Capacitor charging and discharging circuitry



g- 29, 1957 M. c. HOLTJE 3,339,108

CAPACITOR CHARGING AND DISCHARG ING CIRCUITRY Filed Jan. 28, 1965 GATE DELAY GENERATOR MONOSTABLE Has MENTOR MALCOLM C. HOLTJE ATTORNEYS United States Patent 0,

r 3,339,108 CAPACITOR CHARGING AND DISCHARGING CIRCUITRY Malcolm C. Holtje, Concord, Mass., assignor to General Radio Company, West Concord, Mass., a corporation of Massachusetts Filed Jan. 28, 1965, Ser. No. 428,654 16 Claims. (Cl. 315-200) The present invention relates to electric-discharge apparatus and, more particularly, to circuits for supplying electric energy to storage means, such as capacitors, which may be discharged through discharge devices such as a gaseous-discharge lamp as of the strobotron tube type, to produce a flash of light.

In the operation of strobotron or similar tubes there is a fundamental limitation upon the maximum rate at which flashing can occur; namely, the de-ionization time of the tube. While this time can be controlled to some eXtent by the design of the tube, it remains a limit upon the maximum flashing rate. If more than a certain critical voltage is applied across the tube before de-ionization has occurred, continuous conduction (termed holdover) results, and normal flash-tube operation is precluded. In certain types of flash tube circuits, moreover, the current which flows during holdover may quickly destroy the flash tube unless the circuits are complicated by the use of a protective device. Simple flash-tube circuits heretofore employed have thus been incapable of approaching the theoretical flash rate limit, which is the inverse of the de-ionization time.

It is accordingly a principal object of the invention to provide improved flash tube circuits, and especially circuits for supplying electric energy to tubes of the strobotron type.

Another object of the invention is to provide improved circuits for supplying electric energy to capacitor and similar storage devices.

A further object of the invention is to provide improved circuits of the foregoing type which operate from relatively low supply voltage so that solid state switching devices can be readily employed.

Still another object of the invention is to provide circuits of the foregoing type which take advantage of resonant charging effects without the disadvantages of prior circuits of this type.

An additional object of the invention is to provide improved circuits for supplying electric energy to gaseous discharge devices and the like and in which holdover is prevented by ensuring that the supply voltage remains below the holdover potential threshold for an interval at least as great as the de-ionization time.

Another object of the invention is to provide circuits of the foregoing type which are adjustable over a wide operating frequency range without substantially increasing peak currents or operating voltages and with minimal variation of the circuit parameters.

An additional object of the invention is to provide an improved circuit for charging an electric storage device incrementally to a desired level.

A still further object of the invention is to provide a flash-tube supply circuit having an adjustable capacitance to obtain maximum light output over a wide range of flash rates and requiring no other adjustments for proper operation.

3,339,108 Patented Aug. 29, 1967 Additional objects are to provide flash-tube circuits capable of exceptionally high flash rates, approaching the theoretical limit, and which operate reliably at such rates; to provide flash-tube circuits of unusually high efiiciency; and to provide flash-tube circuits and the like which do not require large inductances, even for low frequency operation, and still provide regulated output voltage.

Briefly stated, and without intent to limit the scope of the invention, in one embodiment, the apparatus of the invention comprises a flash tube as of the strobotron type having capacitor means connected across its principal electrodes for supplying flash-producing electric energy. The capacitor means is charged through a transformer having a primary winding onnected to a source of electric energy and to a solid state switch for controlling the application of such energy to the transformer and a secondary winding connected to the capacitor means to form a resonant charging circuit including a charging rectifier. The triggering electrode of the tube is connected to a trigger circuit which also triggers a mono-stable circuit for supplying a pulse to the solid state switch to render the latter conductive. The pulse duration is made equal to or slightly greater than the de-ionization time of the tube. The charging diode is poled to block the voltage produced in the secondary winding during the switch conduction but to pass the current produced during the collapse of the magnetic field in the transformer following the pulse. A regenerative feedback circuit re-triggers the monostable circuit after a delay causing the production of repetitive pulses after the monostable circuit is initially triggered. Each monostable pulse continues to charge the capacitor means incrementally. When the capacitor means has been charged to the desired level, the feedback is automatically interrupted to terminate operation of the monostable circuit.

The foregoing and other objects, advantages, and features of the invention, and the manner in which the same are accomplished will become more readily apparent upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments of the invention in contrast to the prior art, and wherein:

FIG. 1 is a schematic diagram of a common type o strobotron or similar flash-device supply circuit of the type heretofore employed;

FIG. 2 is a graph illustrating the operation of the circuit of FIG. 1;

FIG. 3 is a schematic diagram of another type of priorart strobotron or similar flash-device supply circuit for obtaining better performance at higher flash rates;

FIG. 4 is a graph illustrating the operation of the circuit of FIG. 3;

FIG. 5 is a schematic circuit diagram of an improved flash supply circuit embodying a preferred embodiment of the present invention;

FIG. 6 is a graph illustrating the operation of the circuit of FIG. 5;

FIG. 7 is a similar schematic diagram of a more refined circuit in accordance with the invention for providing simplified operation over a wide frequency range; and

FIG. 8 is a graph illustrating the operation of the circuit of FIG. 7.

Referring to the drawings, FIG. 1 illustrates the most common circuit heretofore employed for supplying power to a strobotron tube or the like, or similar flash device. A capacitor C (or capacitors or other storage device) connected across the principal electrodes of the tube FT 1s charged through a resistor R and is periodically discharged through the tube upon the application of a pulse to the triggering electrode of the tube from a trigger circuit 2. In the graph of FIG. 2 wherein voltage is plotted along the ordinate and time along the abscissa, E represents the supply voltage, E is the minimum desired operating voltage, E the critical holdover voltage, I the de-ionization time of the tube FT, and t the time for the capacitor C to reach E The maximum frequency f at which the tube FT can be flashed with the desired output is If E is to be approximately E, then I 3RC From elementary circuit theory it can be shown that t 3RC=4290 psec.

and

If the operating frequency exceeds 233 c.p.s., the operating voltage and, consequently, the light output per flash will decrease. In the representative flash-tube circuit of this type, if the flash rate is raised to 400 c.p.s., E decreases to 82.5% of its normal value. This is about the maximum frequency for reliable operation. For occasional tubes, operation is possible up to 1,000 c.p.s., where E,, is down to 50% of normal and the light output per flash is about of normal (energy varying as the square of the voltage, but efficiency dropping at lower voltages, also).

At these low operating voltages, the tube becomes very difficult to trigger and the possibility of a misfire exists. If the tube does not discharge the capacitor completely, some charge will remain, and the following charging cycle will start with voltage on the capacitor, which will increase the voltage at time t ab ove E When this happens, holdover results. The holdover current through the flash tube is limited to a safe value by charging resistor R. Operation, however, cannot be resumed until E is removed or reduced below E A well-known technique for obtaining better performance at higher flash rates involves replacing the charging resistor R with an inductor L and a diode D, as shown in FIG. 3. The capacitor now charges through the inductor L and the diode D. The charging curve is illustrated in the graph of FIG. 4, having the primary advantage of improvement in the slope of the charging curve. The fact that this charging curve starts at zero slope helps to keep the voltage low up to time t while the steep portion of the curve represents the transfer of energy to the capacitor quite rapidly. The final voltage E moreover, is twice B, because of the resonant charging possible with the inductance L, so that the supply voltage E can be one-half the value required in the resistor charging circuit of FIG. 1. The diode D prevents the capacitor from discharging back to E with the normal LC oscillation of the circuit. Again, by the application of simple circuit theory, it can be shown With this representative flash tube circuit, the maximum flashing frequency f is Using the technique of FIG. 3, thus, the flash tube FT of the previous example can be operated up to 963 c.p.s. with its full output of five watts. This would require E =1000 volts 5 watts= /2CE f cosand

While such an inductive resonant charging system provides much better operation at high flash rates, the actual maximum flash rate, however, is not greatly improved. The maximum rate is still far from the theoretical limit of l/t There are also other attendant disadvantages. If, for example, the strobotron or similar device is flashed at a rate above f there will be some energy left in the inductor L from the previous flash. The voltage across the capacitor C will, therefore, rise faster, and at time t will be greater than E This will produce holdover. In this circuit, there is nothing to limit the current when holdover occurs, and the flash tube will be quickly destroyed unless some protective device is used. The inductor L that is necessary, furthermore, is quite large and will require larger and larger peak currents from the supply as the flash capacitor is increased (for full output at lower flash rates) unless the inductor is switched in value as the capacitance is changed.

In accordance with the present invention, however, these disadvantageous features and limitations are largely obviated. FIG. 5, for example, illustrates a circuit constructed in accordance with the invention and which is capable of operation at flash rates approaching the theoretical maximum of l/t without the foregoing disadvantages. This circuit, moreover, has the advantage that the supply voltage E can be of an appropriate value, so that low voltages compatible with transistors can be used. In the illustrated embodiment of FIG. 5, a transformer T is provided having a primary winding of inductance L and a secondary winding of inductance L the turns ratio being designated lzn, where n has any appropriate value. The polarity of the windings is indicated by the adjacent dots. The secondary winding L, is connected to the storage means, illustrated as capacitor C, to form a resonant charging circuit including a charging diode D, the capacitor C being connected across the principal electrodes of the flash device FT, as before. A solid state switch S, shown as a transistor, is arranged to complete a circuit through the primary winding L from the source E of DC potential. With an NPN transistor having its collector connected to the lower end of the primary winding L and its emitter connected to ground or chassis potential --E, the positive terminal +E of the DC supply being connected to the upper end of the primary winding L the base of the transistor switch S is connected to a pulser 4 which produces positive pulses having a duration equal to or slightly greater than the de-ionization time I The pulser 4 is actuated by the trigger circuit 2 which also triggers the strobotron FT to discharge the capacitor, as before described.

The operation of the circuit is as follows: Assume that the flash device FT has just been flashed and that the capacitor C is completely discharged. The normally nonconducting or off switch S is rendered conductive or turned on by a pulse of duration t from the pulser 4. This will induce a voltage nE in the secondary winding L of the transformer T. Because of the winding polarity, this voltage back-biases the diode D and no current flows into the storage capacitor C. Thus the voltage across the tube FT remains zero for a period of time equal to t During this time, the current in the primary winding L increases linearly to The stored energy in the primary L reaches 2 2 aLa =g i =aoaa and Et nE't where L =n L Thec-harging time is The capacitance of the storage capacitor C is determined by the ratings of the strobotron tube at the maximum flash rate. The charging time l -t however, can be made as small as desired by reducing L If L is halved, and the turns ratio n is held constant, L will also be halved. To keep the stored energy constant, B mus-t be reduced by the peak current will increase by 2 The maximum flash rate is then It is thus possible to approach the maximum theoretical flash rate and yet to avoid holdover.

For operation over a wide frequency range the flash device FT cannot be used near its maximum rating unless the discharge capacitor C can be changed and several ranges provided. In a representative practical circuit, four six-to-one ranges with a capacitor variation of 2l6-to-l may be provided. For lower frequency operation, the discharge capacitorC may be switched to a larger value and the energy per flash increased proportionately. With a resonant energy transfer of the type just described, the energy stored in the transformer T must also increase proportionately. Some circuit change, accordingly, would appear to be necessary.

If the on-time of the switch S and the transformer inductances are increased by the same ratio as the increase in discharge capacitance, all circuit voltages and currents remain the same, and energy per flash is increased, as desired. The increased charging time creates no problem because the time between flashes of the strobotron is increased proportionately for lower flash rates. Changing the transformer inductance over a wide range (for example, 216:1) is not, however, advantageous.

The increased energy per flash can also be obtained either by increasing the supply voltage or by increasing the on-time of the switching device S by the square root of the ratio of the capacitance increase (i.e., by 15:1).

6 These changes cause the peak current to increase by the square root of the capacitance ratio change (i.e., by 15: 1). Limitations of the solid state switching device S make such current or voltage increases undesirable.

A novel technique for storing more energy per flash without requiring changes in the supply voltage, current, or the transformer is thus provided in accordance with a further feature of the invention illustrated in FIG. 7. The parts corresponding to parts illustrated in FIG. 5 have been designated by the same reference characters, and the corresponding connections need no further description. A monostable circuit or pulser 6 which supplies the pulses of duration r for turning on switch S, is triggered by the same circuit 2 that triggers the strobotron FT and has a regenerative feedback loop FL including a delay device and a gate. The gate 8 is controlled by a flip-flop or bi-stable circuit 10, set in one state in response to the trigger and set in the other state in response to the charging of the capacitor C to the desired potential. For the latter purpose, an avalanche breakdown device, such as a Zener diode D is employed. The Zener diode D is connected between the set input of the flip-flop 10 and the junction of the transistor collector and the lower end of the primary winding L The operation of the circuit of FIG. 7 is as follows: A pulse from the trigger 2 flashes the strobotron tube FT which discharges the storage means, such as capacitor C Simultaneously, the trigger 2 applies a pulse to the monostable circuit 6 causing it to generate a pulse of duration r and also resets the flip-flop 10 so that the gate 8 is open. The pulse from the monostable circuit 6 turns the transistor switch S on for a time t During this time, the diode D is back-biased, and no voltage appears across thertube FT, permitting the tube FT to de-ionize completely. At the end of the pulse t the transistor S is switched off, and the energy stored in the transformer T is resonantly transferred to the capacitor C. As shown in the charging curve of FIG. 8, voltage on the capacitor C is raised incrementally to a value 2 which may be substantially less than the desired operating voltage E The capacitor charges for a time set by the delay generator 12. At the end of this delay time a pulse passes through the open gate 8 to the input of the monostable circuit 6 to cause the same to generate a second pulse of duration t which again turns on the transistor S and preliminarily stores energy in the transformer T. The delay is set to provide an interval between the pulses r to permit time for the transfer of energy from the transformer T to the capacitor C. After the second pulse r the potential on the capacitor C is raised to level e as shown in FIG. 8. Successive pulses are generated by the monostable circuit 6 in the same manner, the termination of each delay pulse initiating the production of the next monostable pulse. This action continues until the voltage across the capacitor C'reaches the desired value E This voltage is determined by the Zener diode D Since the secondary voltage is related to the primary voltage by the turns ratio, the capacitor voltage can be determined in the lower voltage primary circuit. When the primary circuit exceeds the Zener diode voltage, the diode D conducts and sets the flip-flop 10, which closes the gate 8. No more pulses are then generated by the monostable circuit 6 and the system remains at rest with the capacitor C fully charged until an input trigger pulse causes the device FT to flash, resets the flip-flop 10 and starts the monostable circuit 6 again.

The circuit operation is essentially the same regardless of the value of the capacitor. As the capacitor is changed to get maximum light output from tube PT, the number of charging increments changes, but the operation always continues until the capacitor C is charged to the desired value E No changes in the circuit are required.

As the capacitance is increased, the fixed delay time may become insufiicient to permit complete transfer of energy from the transformer T to the capacitor C during each transfer interval, and some current may be flowing in the transformer T at the start of the next transfer interval. This, however, does not alfect the operation of the circuit. During the next transfer interval, the current from the preceding interval is merely added and the energy transfer per cycle increased. The only resulting effect is an increase in the peak current through the switching device S. If this becomes undesirable, a current-limiting device in this circuit will keep the peak charging current fixed. A few extra charge increments will then be required to reach the desired final capacitor operating voltage.

Holdover in this circuit is not possible so long as the duration of the pulses t is at least as great as the deionization time of the tube, because no increase in voltage across the discharge capacitor can occur for at least 2 seconds after a discharge.

The circuit of the invention operates with suitable strobotron and related tubes at a frequency of 7,000 c.p.s., approaching the maximum theoretical operating frequency of the tube and the maximum theoretical efficiency. A low supply voltage may be employed, making feasible the use of a transistor switch S, and a small transformer may be used (for example, L =3 mh. and L =l.2 h.). The output voltage is closely regulated by the Zener diode D While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes can be made without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims. Certain advantages of the invention can be obtained with different components. For example, within the broader aspects of the invention, a simple inductor, rather than a transformer, may be used. However, if a transistor switch is employed to control the flow of current into the inductor from the source E, the source voltage will have to be compatible with the transistor. Accordingly, the foregoing embodiments are to be considered illustrative, rather than restrictive of the invention, and those modifications which come within the meaning and range of equivalency of the claims are to be included therein.

What is claimed is:

1. Apparatus having, in combination, a gaseous discharge flash tube, a capacitor connected to said tube to supply electric energy for the discharge, a rectifier, an inductance connected to said capacitor through said rectifier, and means for applying a pulse of electric energy to said inductance, said rectifier being poled to block the voltage induced in said inductance during said pulse but to pass current from said inductance to said capacitor during collapse of the magnetic field of said inductance after said pulse, the duration of said pulse being at least as great as the de-ionization time of said tube.

2. The apparatus of claim 1, said inductance comprising the secondary winding of a transformer having a primary winding to which said pulse is applied.

3. The apparatus of claim 2, said pulse applying means comprising a solid-state switch connected to said primary winding.

4. The apparatus of claim 1, said pulse applying means comprising a pulser, and means for causing said pulser to produce repetitive pulses until said capacitor has been charged to a predetermined potential.

5. The apparatus of claim 1 further comprising means for triggering said tube to produce a flash, said pulse applying means being controlled by said triggering means.

6. The apparatus of claim 5, said pulse applying means comprising a pulser responsive to said triggering means for generating a pulse of duration at least as great as the de-ionization time of said tube, means for causing said pulser to generate another such pulse a predetermined time after the end of the preceding pulse and thereafter to generate such pulses repetitively, and means responsive to the attainment of a predetermined potential on said capacitor for terminating the generation of such pulses.

7. The apparatus of claim 6, said causing means comprising a regenerative circuit connecting the output and the input of said pulser and including a delay device and a gate, said terminating means comprising a bistable device for controlling said gate, said bistable device being set in one state in response to said triggering means and in its other state in response to the capacitor potential.

8. The apparatus of claim 7, there being provided an avalanche breakdown device for setting said bistable device in said other state in response to the capacitor potential.

9. Apparatus for charging an electric storage device by increments until a predetermined potential is attained, comprising preliminary means for storing a pulse of electric energy and for thereafter transferring said energy to said storage device, a pulser having an input and an output, means connecting the output of said pulser to said preliminary storage means, means including a pulse delay device for connecting the output of said pulser to the input of said pulser regeneratively, and means responsive to the level of the electric energy in said storage device for interrupting the connection between the output and the input of said pulser.

10. The apparatus of claim 9 further comprising means for discharging said storage device.

11. The apparatus of claim 10 and in which means is provided for commencing the generation of pulses by said pulser upon the discharging of said storage device.

12. The apparatus of claim 11 and in which said connecting means comprises a gate, said means for commencing the generation of said pulses comprising means for opening said gate, said means for interrupting said connection comprising means for closing said gate.

13. The apparatus of claim 12 and in which said pulser comprises a monostable device and said means for opening and closing said gate comprising a bistable device.

14. Apparatus comprising, in combination, a flash tube having a pair of discharge electrodes, a capacitor connected between said electrodes, a transformer having a primary winding and having a secondary winding connected across said capacitor through a rectifier, a source of electric energy, means including a solid state switch for completing a circuit from said source through said primary winding, monostable pulse producing means for applying a pulse to said switch to render said switch conductive and to produce a pulse of electric energy in said secondary winding, said rectifier being poled to block the induced voltage during said pulse of electric energy but to pass a current during the collapse of the field in said transformer after said pulse, means for triggering said flash tube and said monostable means, a regenerative feedback circuit connecting the output of said monostable means and the input thereof and including delay means and gate means, and flip-flop means for opening said gate means upon the triggering of said monostable means and for closing said gate means when said capacitor has been charged to a predetermined potential.

15. The apparatus of claim 14, said flip-flop means having means connected to said primary winding for determining when said capacitor has been charged to said potential.

16. The apparatus of claim 15, the last-mentioned means comprising an avalanche breakdown device.

References Cited UNITED STATES PATENTS 2/1966 Sokolov 3l5-209 4/1966 Tomkinson 315241 

1. APPARATUS HAVING, IN COMBINATION A GASEOUS DISCHARGE FLASH TUBE, A CAPACITOR CONNECTED TO SAID TUBE TO SUPPLY ELECTRIC ENERGY FOR THE DISCHARGE, A RECTIFIER AN INDUCTANCE CONNECTED TO SAID CAPACITOR THROUGH SAID RECTIFIER, AND MEANS FOR APPLYING A PULSE OF ELECTRIC ENERGY TO SAID INDUCTANCE, SAID RECTIFIER BEING POLED TO BLOCK THE VOLTAGE INDUCED IN SAID INDUCTANCE DURING SAID PULSE BUT TO PASS CURRENT FROM SAID INDUCTANCE TO SAID CAPACITOR DURING COLLAPSE OF THE MAGNETIC FIELD OF SAID INDUCTANCE AFTER SAID PULSE, THE DURATION OF SAID PULSE BEING AT LEAST AS GREAT AS THE DE-IONIZATION TIME OF SAID TUBE. 